GDScriptVM: Avoid c-style cast/const_cast

[rune-scape]

+replaced c-style casts with more verbose and searchable c++ casts
+moved common const_cast of instruction_args to const Variant ** into a ref var

[fire]

What is the benefit for this code style change?

Edited: it's not very long so it's probably ok and localized to gdscript

[rune-scape]

im searching for const_casts after #93759, and trying to find ways to remove them or improve them, this is just a style change. bc of how Vector and CowData might(?) copy the data if stack.ptrw() was used. i didn't want to touch that

the style guide doesnt say anything about casts, but im assuming moving away from c-style casts toward verbose casts is desired